Physical working capacity decreases progressively during adult life. Contributing to this decline are decreases in function of the cardiovascular and pulmonary systems as well as a decrease in muscle mass. It is our hypothesis that decreased ability to perform physical work in the elderly is also associated with changes in the hormonal regulation of blood "fuel" regulation during exercise. The purpose of the present study is to examine the effect of age on plasma levels of glucose, free fatty acids (FFA) and of hormones responsible for regulating energy stores (catecholamines, glucagon, insulin) both at rest and during prolonged exercise. Plasma levels of glycerol, triglycerides and lactic acid are also measured. Exercise is accomplished by progressively increasing levels of treadmill walking. Subjects walk for 10 minutes at each of 3 exercise levels, 40, 50 and 60% of their previously determined maximal oxygen consumption (VO2max), and for 30 minutes at 70% of VO2max. Thus, duration of an exercise session is 1 hour. Blood samples for metabolite and hormone determinations are taken before exercise, at the end of each exercise stage, and 15 minutes after its termination. Samples of expired air are obtained before exercise, during the final 2 minutes of each submaximal workload and at the end of the recovery period for measurements of minute ventilation, oxygen consumption, carbon dioxide production, and respiratory quotient. The ECG is monitored continuously during the entire experimental period. Healthy subjects are selected from participants in the Baltimore Longitudinal Study of Aging according to the following criteria: no evidence of cardiovascular or other significant disease, negative treadmill maximal exercise test, no cardioactive medication, no cigarette smoking, non-obese and normotensive. Twenty-four men covering the adult age span have completed the protocol. Results will be analyzed in the coming year. Preliminary results on serum testosterone levels (basal and at the end of exercise) in the first 19 men show that both original levels and responses are highly positively correlated with the level of physical fitness (VO2max) of the participants.